There is known a polygon mirror (rotary polyhedron) as an actuator provided in laser printers, for example. In such a printer, in order to achieve higher-resolution and higher-quality printed output as well as higher-speed printing, it is necessary to rotate the polygon mirror at higher speed. Currently, an air bearing is used to rotate the polygon mirror at high speed with stability. However, the method using the air bearing involves a problem in that it is difficult to rotate the polygon mirror at much higher speed than the speed available at the present. Further, although a larger motor is required in order to rotate the polygon mirror at higher speed, use of such a larger motor gives rise to a problem in that it is difficult to miniaturize the size of an apparatus in which the polygon mirror is used. Furthermore, use of such a polygon mirror gives rise to another problem in that the structure of the apparatus becomes necessarily complicated, thus leading to an increased manufacturing cost.
Taking such problems into account, an actuator of a relatively simple structure has been proposed that employs a torsional vibrator (for example, see JP-A No. 2004-13099). The actuator disclosed in this reference includes a fixed comb electrode and a movable comb electrode, both of which mesh with each other in a spaced-apart relationship. In the actuator of this type, alternating voltages are applied to between the fixed comb electrode and the movable comb electrode, thus creating an electrostatic attractive force to rotate the movable electrode. In the actuator referred to above, a SOI substrate having a SiO2 layer sandwiched between two silicon layers is used. The movable comb electrode is patterned on one of the silicon layers of the SOI substrate, while the fixed comb electrode is patterned on the other silicon layer of the SOI substrate. Thus, the fixed comb electrode and the movable comb electrode remain out of alignment with each other in a thickness direction of the SOI substrate. This enables the actuator to start its operation in an easier manner.
According to the above-described actuator, however, it is necessary to form masks on the opposite surfaces of the SOI substrate by use of a photoresist and then to etch both of the silicon layers, at the time of forming the fixed comb electrode and the movable comb electrode. For this reason, the gap between the fixed comb electrode and the movable comb electrode varies depending on the alignment accuracy of the masks formed on the opposite surfaces of the SOI substrate, thereby making it difficult to provide the gap with an increased degree of precision. As a result, the gap between the fixed comb electrode and the movable comb electrode becomes greater, which results in a problem in that an increased drive voltage is required.